The invention is particularly applicable for inductively heating exhaust valve seats or valve seat inserts of internal combustion engines and will be described with reference thereto; however, the invention, in its broader application, may be used in other heat treating applications wherein accurate positioning of an inductor coil is required to assure uniform heating of the surface to be treated, notwithstanding manufacturing variations in the location of the surface and misalignment in the presentation of the inductor coil to the surface to be heated.
The conical valve seats for engine heads are commonly quench hardened to provide extended wear characteristics at the poppet valve-valve seat interface. Inductive heating of the interface has become widely accepted in achieving this objective. For satisfactory heating and subsequent hardening, the inductor coil must be accurately located adjacent the seat, energized by a high frequency current to raise the surface to the elevated heat treating temperature and thereafter rapidly cooled or quenched. Various parameters must be controlled to achieve the uniformity. First, the inductor must be formed complementary to and accurately located with respect to the valve seat surface to insure a uniform width inductive airgap which will result in a uniform magnetic coupling with the coil and, consequently, uniform heating. It is also necessary to attain such relationships in mass production for economical manufacture of motor vehicle engines. This accuracy is difficult to attain unless the inductor positioning apparatus can account and compensate for manufacturing variations in the radial and axial location of the valve seat together with the variations in orientations of the valve seat relative to the heating apparatus as the components traverse past the heating station on a conveyor line. Inasmuch as the apparatus is generally fixedly located with respect to the conveyor line, its operation is mechanically constrained by the apparatus. Each engine component is mounted on the suitable fixture carried by the line. Thus, variations between the fixture and the component, the fixture and the line, and line orientation on a component to component basis with respect to the apparatus, can result in cumulative angularity variances. This need for positioning accuracy has resulted in various approaches being taken with regard to the positioning of the inductor. For instance, U.S. Pat. No. Re 29,046 illustrates a heat treating apparatus which allows individual inductor assemblies to radially float with respect to the valve seat as they are mechanically axially advanced toward the valve seat area. This radial float is accomplished by inductor assemblies which carry a centering nose which enters the valve stem bore accurately coaxially formed with the valve seat. The ability to move in a radial plane or a plane transverse to the axis of the inductor coil allows the inductor to be mechanically positioned as the inductor nears the interface. This is highly effective in accomodating manufacturing variances in the spacial location of the valve seat. Such approach is ideally suited for the situation where the mechanical axis of the coil is parallel to the axis of the valve seat. In other words, the ability of the inductor to float in a plane perpendicular to both axes will result in self-centering of the coil with respect thereto. Other approaches for achieving this radial alignment feature are illustrated in U.S. Pat. Nos. 4,266,109; 3,837,934; 3,777,096; 3,761,669; 3,743,809 and 3,737,612. Having achieved the concentric coaxial alignment between the valve seat and the inductor coil as prescribed by the centering action of the nose, it is also necessary to accurately establish the axial location of the inductor with respect to the valve seat. The effective depth of such conical surfaces can vary from valve to valve and engine to engine. This axial positioning must be attained to establish the required inductive air gap, generally in the order of 0.030 to 0.050 inches. In addition to creating a uniform inductive coupling and thereby uniform heating, the position also interrelates with the power control system to insure that the required controlled temperatures are attained, and that, with regard to the simultaneous heating of multiple valve seats, an inductive current balance is provided among the various inductors.
One successful approach, as illustrated in the aforementioned U.S. Pat. No. Re. 29,046, has been to mount a plurality of inductor assemblies on a common frame to move the inductors as a bank toward the valve seat. The radial float capability allows the individual inductors to seat with respect to each seat. However, inasmuch as the depth of the valve seat may vary from seat to seat, the individual inductor assemblies are spring biased and all are allowed to physically engage the valve seat against a spring biasing. After seating of all the inductors, the inductors are individually locked at the frame and the frame withdrawn to prescribe the predetermined axial distance corresponding to the desired inductive gap. This repetitively provides accurate inductive positioning for axial and radial variations in a location of the valve seat surface.
Notwithstanding the accomodation of radial and axial variations in valve seat location, the apparatus does not inherently compensate for non-parallelism between the axis mechanically prescribed by the apparatus and the axis of the valve seat as presented relative to the machine at the heat treating station. While efforts have been made to limit such angularity by careful control of the manufacturing and production process, it has heretofore not been entirely possible to eliminate this misalignment. While this angularity can be partially compensated by part deflection or inherent freedom of movement of the assembly, this puts additional loading on the parts and may cause premature failure of the components. To a large extent, this angularity is compensated by freedom of movement between the centering nose and the valve stem bore. Typically, the centering nose penetrates the valve stem bore a rather short distance, generally in the order of 1/2 to 5/8 inch and has a clearance fit with respect to the valve bore surface. This will allow a certain misalignment or cocking to be tolerated without loading the assembly. However, these clearance relationships may be established at a sacrifice in concentricity between the parts. Further, any tolerated angularity, of necessity, results in a non-uniformity of the coupling gap as the inductor coil is retracted from physical engagement to the heating position. Thus, while substantially improving the accuracy with which valve seat surfaces may be heated, there is nonetheless a need for a device which will compensate for the non-parallelism between the valve seat axis and the inductor coil axis.